MetaStackTracker: eliminate the resynchronization process

The step where we requery the stacking order from the server than combine
it in an arbitrary fashion with Wayland windows can be eliminated by observing
that we are the final authority for Wayland window stacking - so if we
apply each X event that we receive from the X server to our stack in a
way that leaves the X windows ordered in the same way as on the server,
and apply events that we have stored locally in a way that doesn't affect
the ordering of X windows, than we have a fully correct ordering of windows.

Ordering this in the order of first applying the X event and then applying the
local portion also means that as long as we had an up-to-date view of the X
stack the composite operation will be identical to what was requested.

https://bugzilla.gnome.org/show_bug.cgi?id=736559